1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens, and is suitable for, e.g., a broadcasting television camera, movie camera, video camera, digital still camera, surveillance camera, or silver-halide photographic camera.
2. Description of the Related Art
Recently, image pickup apparatuses such as a television camera, movie camera, silver-halide film camera, digital camera, and video camera require a zoom lens having a wide angle of field, a high zoom ratio, and a high optical performance over an entire zoom range. Various zoom lenses having wider angles of field and higher zoom ratios than those of the conventional lenses are known. For example, so-called four-unit zoom lenses are known (Japanese Patent Application Laid-Open Nos. 2001-356381 and 2009-156893). These zoom lenses each include, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit for zooming having a negative refractive power, a third lens unit that corrects an image plane variation caused by magnification-varying and has a positive refractive power, and a fourth lens unit for imaging having a positive refractive power.
Japanese Patent Application Laid-Open No. 2001-356381 has disclosed a four-unit zoom lens having a zoom ratio of about 2.5 and an angle of shooting field at a wide-angle end of about 30°. Japanese Patent Application Laid-Open No. 2009-156893 has disclosed a four-unit zoom lens having a zoom ratio of about 2.7 and an angle of shooting field at a wide-angle end of about 34°.
It is relatively easy to increase the angles of field and zoom ratios of the four-unit zoom lenses having the above-described lens configurations. However, in order to obtain a high optical performance over an entire zoom range by using the four-unit zoom lenses having the above configurations, it is important to well correct, e.g., a lateral chromatic aberration and various off-axis aberrations at the wide-angle end. The lateral chromatic aberration is easily well corrected by using an optical material having anomalous dispersion.
It is, however, difficult to well correct a chromatic aberration even by simply using a lens made of an optical material having anomalous dispersion. On the other hand, it is possible to relatively easily correct the various off-axis aberrations by increasing the number of lenses of each lens unit. However, if the number of lenses is increased, the whole system increases in size and becomes difficult to manufacture.
In order to obtain a high optical performance over the entire zoom range of the above-described, four-unit zoom lens, it is important to appropriately set the lens configuration of the fourth lens unit that is not moved for zooming, and appropriately set the material of each lens included in the fourth lens unit. If these settings are inappropriate, it becomes difficult to correct the lateral chromatic aberration at the wide-angle end, and obtain a high optical performance over the entire zoom range.